A challenge associated with electrostatic voltage metrology is to perform the measurement without disturbing the electrical state of the measured surface/object. This condition can only be accomplished by a device/instrument that has an input resistance of extremely high value, much higher than that of the measured electrostatic system/surface/object being measured. This high input resistance limits the amount of electric charge migrating between the measuring device and the object under test when performing contacting measurements.
Another factor influencing the measuring device's capability for electrostatic voltage measurement is the amount of input capacitance associated with the measurement device's input sensor circuitry, together with the parallel capacitance associated with any connection circuits to the sensor element. The detrimental effect that the input capacitance of the sensor has on electrostatic voltage measurements is with the electrical charge required to be supplied by the measured electrostatic object to drive the input capacitance of the sensor to the measured object's voltage level. The required charge transfer from measured object to sensor will alter the electrical state of the measured object.
Prior art contacting electrostatic voltmeters and devices, i.e., electrometers, typically feature an input capacitance of 1×10−15 farad with input bias currents of 1×10−15 amps with input resistances of 1015 ohms (see U.S. Pat. No. 3,870,968), and so they cannot be used successfully to perform contacting or noncontacting electrostatic voltage measurements without causing substantial distortion to the initial electrical state of the contacted measured object or producing measurement error when making noncontacting measurements. These prior art devices employ buffer amplifiers or operational amplifiers connected as unity gain follower amplifiers, which feature input bias currents in the order of 10−14 amps. If the input capacitance of these devices is lowered to a value in the order of 10−16 farads, the 10−14 input current would produce an uncontrollable input drift rate due to the input current providing a large source into the input capacitance to cause a large
      ⅆ    v        ⅆ    t  drift,where
      ⅆ    v        ⅆ    t  is the voltage change per unit time, and is equal to
  i  cwhere i is the input current and c is the input capacitance, as conventionally known. Additionally, bias current levels of 10−14 amp will transfer a considerable amount of charge to a contacted measured object causing, again, distortion of the measured object voltage level.
The device described in U.S. Pat. No. 3,870,968 also suffers from other disadvantages. These disadvantages include:                1. As conventionally known, the connection of high gain operational amplifier circuitry, including integrated circuitry, as unity gain voltage followers introduces the common mode gain of the amplifier as a limitation on the open loop gain characteristic. An operational amplifier having an open loop gain characteristic of for example, 100 DB (gain=100,000) at DC operating as a single input summing amplifier, but having a common mode gain of 66 DB (gain=2,000) will result in a closed loop follower gain based upon the 66 DB gain when operated as a common mode amplifier, as in the case of U.S. Pat. No. 3,870,968. In addition, the common mode gain characteristic is highly dependent upon operating frequency and can fall to as little as 20 DB (gain=10) at as low a frequency as 10 kHz. As detailed in the cited prior art, the amplifier shown used as a voltage follower has a gain of about 66 DB (gain=2,000). The gain of 2,000 is not adequate to bootstrap the input resistance and capacitance to the 1017 ohm and 1×10−17 farad respectively as required for contacting or noncontacting electrostatic voltmeter service. As will be shown, the invention of this disclosure addresses this issue by providing amplifier circuitry connection which eliminates the common mode operation of the follower connected amplifier, thus allowing the follower connection gain to be equal to the single ended gain characteristic of the amplifier of approximately 100 DB. At a follower connected gain of 100,000 (100 DB) rather than 2,000 (66 DB), the proper input characteristic for contacting or noncontacting electrostatic voltmeter service can be easily achieved. In addition, as the frequency characteristics of an amplifier operating in the common mode connection are considerably lower than the single ended connected operation, the frequency characteristic of the follower using the connection of this invention is much broader than the prior art device due to the bootstrapping of the follower power supply connection, as will be presently explained, thus reducing the various capacitances within the follower's internal circuitry as well as external circuitry, resulting in higher frequency operation capability.        2. The method used by the prior art device to eliminate charge from the input circuitry to cause it to return to a zero potential relative to ground is by use of a switch connecting the protective zener diodes which are connected between the follower's input and the output of an integrating amplifier whose input integrates the voltage differential between the follower output and ground, and then switching the protective zener diodes to be disconnected from the output of the integrating amplifier and reconnected to the follower output. As this technique may be useful when the follower input capacitance is on order of 10−15 farads as claimed by the prior art, at 10−17 farads, as obtained by the invention described herein, the charge transfer through the zener diodes upon switching the diodes between the integrator output and follower output, as used by the prior art device, produces a        
      ⅆ    v        ⅆ    t  across the zener devices to inject a charge into the input where the charge
  (      “                  ⅆ        Q                    ⅆ        t              ”    )injected to the input circuit is equal to
      C    ⁢                  ⅆ        v                    ⅆ        t              ,where C is the capacitance of the series connected zener diodes connected to the input, and
      ⅆ    v        ⅆ    t  is the rate of voltage change at the integrator/output side of the zener diodes caused by switching the protective diodes between the integrator and the follower output. This charge transfer effect when zeroing the prior art device is eliminated by the instant invention.                3. Another disadvantage of the cited prior art device lays with the necessity to intentionally introduce a small voltage offset error between the voltage follower input and the follower output to correspondingly introduce a current into the voltage follower input through the first protective device that is equal to the first gate input current, see U.S. Pat. No. 3,870,968, claim 9. The operation of the follower of the instant invention does not require the introduction of a small voltage offset between the follower input and output, thereby allowing the output to be equal to and accurately follow the measured voltage of the input. In addition, to maintain very high equivalent input resistance, the voltage appearing across any resistive path associated with the input circuitry must be bootstrapped within microvolts of zero as provided by the output/input bootstrapping current of the instant invention, thereby negating the possible use of any intentional voltage offset there-between.        